1. Field of the Invention
The present invention relates generally to methods and apparatus for servicing a well, and more particularly to methods and apparatus for the early evaluation of a well after the borehole has been drilled and before casing has been cemented in the borehole.
2. Description of the Prior Art
During the drilling and completion of oil and gas wells, it is often necessary to test or evaluate the production capabilities of the well. This is typically done by isolating a subsurface formation which is to be tested and subsequently flowing a sample of well fluid either into a sample chamber or up through a tubing string to the surface. Various data such as pressure and temperature of the produced well fluids may be monitored down hole to evaluate the long-term production characteristics of the formation.
One very commonly used well testing procedure is to first cement a casing in the borehole and then to perforate the casing adjacent zones of interest. Subsequently the well is flow tested through the perforations. Such flow tests are commonly performed with a drill stem test string which is a string of tubing located within the casing. The drill stem test string carries packers, tester valves, circulating valves and the like to control the flow of fluids through the drill stem test string.
Although drill stem testing of cased wells provides very good test data, it has the disadvantage that the well must first be cased before the test can be conducted. Also, better reservoir data can often be obtained immediately after the well is drilled and before the formation has been severely damaged by drilling fluids and the like.
For these reasons it is often desired to evaluate the potential production capability of a well without incurring the cost and delay of casing the well. This has led to a number of attempts at developing a successful open-hole test which can be conducted in an uncased borehole.
One approach which has been used for open-hole testing is the use of a weight-set, open-hole compression packer on a drill stem test string. To operate a weight-set, open-hole compression packer, a solid surface must be provided against which the weight can be set. Typically this is accomplished either with a tapered rathole type packer as shown in U.S. Pat. No. 2,222,829 to Humason et al., or with a perforated anchor which sets down on the bottom of the hole. A disadvantage of the use of open-hole compression set type packers is that they can only be used adjacent the bottom of the hole. Thus, it is necessary to immediately test a formation of interest after it has been drilled through. These types of packers cannot be utilized to test a subsurface formation located at a substantial height above the bottom of the hole. Also, this type of test string is undesirable for use offshore because the pipe string can become stuck in the open borehole due to differential pressures between the borehole and various formations. As will be understood by those skilled in the art, when the pipe string is fixed and is no longer rotating, portions of the pipe string will lie against the side of the borehole and sometimes a differential pressure situation will be encountered wherein the pipe string becomes very tightly stuck against the side wall of the borehole. This is especially a dangerous problem when the flow control valves of the test string are operated by manipulation of the test string. In these situations, if the test string becomes stuck it may be impossible to control the flow of fluid through the test string.
Another prior art procedure for open-hole testing is shown in U.S. Pat. No. 4,246,964 to Brandell, and assigned to the assignee of the present invention. The Brandell patent is representative of a system marketed by the assignee of the present invention as the Halliburton Hydroflate system. The Hydroflate system utilizes a pair of spaced inflatable packers which are inflated by a downhole pump. Well fluids can then flow up the pipe string which supports the packers in the well. This system still has the disadvantage that the pipe string is subject to differential sticking in the open borehole.
Another approach to open-hole testing is through the use of pad-type testers which simply press a small resilient pad against the side wall of the borehole and take a very small unidirectional sample through an orifice in the pad. An example of such a pad-type tester is shown in U.S. Pat. No. 3,577,781 to Lebourg. The primary disadvantage of pad-type testers is that the take a very small unidirectional sample which is often not truly representative of the formation and which provides very little data on the production characteristics of the formation. It is also sometimes difficult to seal the pad. When the pad does seal, it is subject to differential sticking and sometimes the tool may be damaged when it is removed.
Another approach which has been proposed in various forms, but which to the best of our knowledge has never been successfully commercialized, is to provide an outer tubing string with a packer which can be set in a borehole, in combination with a wireline-run surge chamber which is run into engagement with the outer string so as to take a sample from below the packer. One example of such a system is shown in U.S. Pat. No. 3,111,169 to Hyde, and assigned to the assignee of the present invention. Other examples of such devices are seen in U.S. Pat. Nos. 2,497,185 to Reistle, Jr.; 3,107,729 to Barry et al.; 3,327,781 to Nutter; 3,850,240 to Conover; and 3,441,095 to Youmans.
The present invention provides a number of improvements in open-hole testing systems of the type generally proposed in U.S. Pat. No. 3,111,169 to Hyde.